1. Field of the Invention
This invention relates generally to improvements in well drilling system and apparatus, and more particularly to downhole drilling motor apparatus having a bent or curved drive section.
2. Brief Description of the Prior Art
It has been recognized that a number of advantages can be gained in drilling wells by employing a stationary drill pipe or drill string which has attached at its lower end a downhole motor, the drive section of which is connected to and rotates a drill bit. In such apparatus a fluid, e.g., air, foam, or a relatively incompressible liquid, is forced down the stationary drill pipe or drill string and on passing through the fluid-operated motor causes rotation of a shaft ultimately connected to the drilling bit. The drill string is held or suspended in such a manner that it does not rotate and therefore may be regarded as stationary. However, it is lowered in the well as drilling proceeds.
The forces required to rotate the rotary bit at the bottom of the string are such that in the usual situation the fluid operated motor must be quite lengthy. Conventional straight hole drilling motors such as the Moineau (Moyno) motor comprise three sections, the rotor/stator section which contains a rubber stator and a steel rotor; the universal section which contains the universal joint or flexible connection that converts the orbiting motion of the rotor to the concentric rotary motion of the bit; and the bearing pack section which contains radial and thrust bearings to absorb the high loads applied to the drill bit. The rotor/stator section of the motor is typically 2-3 times longer than the bearing pack section.
In directional drilling, drilling motors of this general character are utilized wherein a bend may be located in the drill string above the motor, a bend may be placed in the motor housing below the rotor/stator drive section, or the bit or output shaft is angularly offset relative to the drive section axis.
In some directional drilling systems, such as Dellinger al, U.S. Pat. No. 4,577,701 and British Patent 1,494,273, the practice has been to position a "bent sub" between the top of the fluid-operated motor and the axis of rotation of the bit to the axis of the drill pipe. However, due to the length of the motor required and other structure connecting the rotor of the motor to the bit, the spacing of the "bent sub" from the bit is excessive. This distance frequently amounts to approximately 22 feet or more which is objectionable due to the fact that it is difficult to position and to maintain the orientation of the bit in relation to the axis of the drill pipe.
In an attempt to overcome this problem other systems have been designed to place the bend closer to the bit, such as Nielson et al, U.S. Pat. No. 3,260,318.
These types of systems modify the universal section of the drilling motor. Because the lower end of the rotor in the aforementioned types of motors gyrate about the axis of its stator, some form of universal joint or flexible connection is employed in the driving connection between the rotor and the bit which rotates about a stationary axis. As a clearance must exist between this universal joint or flexible connection and the walls of the surrounding housing to accommodate the flexibility of movement, a bend is formed in the housing of the universal section of the motor. In this manner, the axis of rotation of the bit is angularly related not only to the axis of the drill string but also the axis of the fluid-operated motor. This aids in obtaining and maintaining control and orientation of the bit. However, placing the bend in the housing surrounding the universal joint limits the severity of the bend which can be used.
Other systems, such as Henderson, U.S. Pat. No. 3,667,556 and Kamp, European Patent 109,699 disclose apparatus wherein the drill bit or output shaft is angularly offset relative to the motor drive section axis. Applicant's prior patent having the same assignee as this application, Maurer et al U.S. Pat. No. 4,823,053 shows an apparatus substantially the same as in this invention in which the apparatus is for drilling wells with a curvature of 200-2000 feet.
Combinations of the above described prior art systems may also be used in directional drilling, however none utilize downhole fluid motors having a bent or curved rotor/stator section. Because the rotor/stator section of the motors are typically 2-3 times longer than the bearing section, the prior art systems are not particularly suitable for use in drilling high curvature horizontal wellbores, such as medium-radius (10 to 1,000 feet), from vertical or near vertical wells. The present invention may be used in conjunction with the prior art apparatus such as bent subs, bent housings, and inclined motor shafts.
The present invention is distinguished over the prior art in general, and these patents in particular by a system which utilizes a downhole fluid-operated motor having a bent or curved rotor/stator drive section which allows it to be used in high curvature wellbore applications without interfering with the wall of the borehole.